Dementia and other forms of cognitive impairment are major health problems in the USA and other countries. Clinical studies provide invaluable information about mechanisms, pathology and clinical course, but there is an urgent need for high quality population data concerning the prevalence of dementia and mild cognitive impairment in order to plan services, predict future needs, estimate the costs of dementia care, and understand the impact of these conditions on individuals and their families. Embedding work on dementia into longitudinal studies of aging populations is particularly advantageous, since it permits investigations of cognitive, biological, and social antecedents and consequences of the development of these problems. Cross- national comparisons of dementia and cognitive impairment within harmonized longitudinal aging studies provide invaluable evidence contributing to understanding of the processes underlying national differences, and are crucial in determining the appropriate policy response. The English Longitudinal Study of Ageing (ELSA) is the most mature of the sister studies to the Health and Retirement study, since it has been in progress since 2002 and has already collected 6 complete waves of data collection. The studies are closely harmonized in terms of social, economic, behavioral and genetic data, so are well placed for making direct comparisons of dementia and mild cognitive impairment in England and the USA. The purpose of this application is to collect detailed data in ELSA on cognition and related processes on a subset of the ELSA population using an enhanced assessment protocol harmonized with measures that will be administered in the HRS. The assessment will include more extensive cognitive measures than are collected in regular waves of ELSA together with interviews with informants on a stratified random subsample of 1,000 ELSA participants aged 65 and older. In collaboration with the HRS team, these will be used to develop algorithms to classify dementia, mild cognitive impairment and healthy cognitive function. Comparisons between these measures and the regular cognition measures in ELSA will allow extrapolation to the ELSA population in general. We will then be able to make direct comparisons of the prevalence of these problems in ELSA and the HRS. We will also carry out studies of the validity of dementia estimates by carrying out back-validation against data collected in the Medical Research Council Cognitive Function and Ageing Study, a leading UK population study of dementia, and by testing associations within ELSA between the new measures and the objective clinical data obtained through UK Hospital Episode and Primary Care Statistics. Another important strand of work will involve quantifying the impact of attrition from ELSA on estimates of cognitive impairment, using a number of methods to triangulate on relevant differences between people who remain in the study and drop out. The dataset will provide invaluable material for future analyses of factors predicting cognitive impairment and of the social and economic consequences of dementia.